1. Field of the Invention
The present invention relates to a transparent camera calibration tool that is employed in camera calibration of computer vision for estimating parameters of six degrees of freedom (extrinsic parameters) for the position and orientation of a camera and parameters (intrinsic parameters) such as the image center position and lens distortion coefficients, which are camera-specific qualities, and to a geometrical calibration method that employs the transparent camera calibration tool.
2. Description of the Related Art
In computer vision, the estimation of extrinsic parameters, which express the relationship between a camera coordinate system and a world coordinate system and intrinsic parameters, which express the relationship between an image coordinate system and the camera coordinate system, is indispensable. It is difficult to determine the position and orientation of a camera on the basis of camera specifications, mounting position information, and so forth, and to determine the focal length from the lens calibration, and so forth, when accuracy is lacking in a real operation and, therefore, generally, camera calibration that entails imaging a calibration tool whose size is already known once and estimating the parameters for the camera position and orientation based on this image is performed. FIG. 20 schematically shows a camera calibration method that uses a conventional calibration tool. First, a calibration tool is disposed close to the object and imaged by means of the camera as a preprocessing to capturing image as shown above on the right, whereupon an image of the calibration tool as shown on the center right is obtained and analyzed and the extrinsic and intrinsic parameters of the camera are calculated and stored. A procedure is adopted in which the calibration tool is withdrawn a certain time afterward, the object is captured as illustrated above on the left to obtain an observed image of the object as shown on the center left, whereupon a three-dimensional analysis of the observed image is carried out on the basis of the previous camera extrinsic and intrinsic parameters. Because an analysis of the observed image is performed on the basis of the camera extrinsic and intrinsic parameters obtained from the calibration tool image in this manner, the position and orientation of the camera and the focal length thereof, and so forth, cannot be changed following calibration.
In the method that is disclosed in Japanese Patent Application Laid Open No. H8-86613 “Stereo camera calibration device” (Apr. 2, 1996), a perforated board c that is rendered by forming a multiplicity of holes b in the upper side of a plate a as shown in FIG. 19 is provided and calibration poles d of rod material are randomly fitted in optional positions of the multiplicity of holes b in the perforated board c. The upper side of plate a is coated black, the upper side of perforated board c is coated gray, and the tops of the calibration poles d are coated white. Further, the lengths of the calibration poles d are set at random. Two cameras (a left camera and right camera) e and f are arranged on top of a calibration table in a cross-eye view so that the optical axes of the left camera e and right camera f are roughly joined at a certain point on the calibration table. An image of the upper side of the calibration table is taken by left camera e and right camera f. The images taken by the left camera e and right camera f are inputted to a coordinate circuit and coordinates are established by the coordinate circuit on the basis of the shading of the images. The information of the coordinate circuit is then inputted to a three-dimensional position processing circuit and calibration parameters estimated by a calibration parameter processing circuit are inputted to the three-dimensional position processing circuit. The three-dimensional position processing circuit determines a three-dimensional position of a projected point based on the information of the coordinate circuit and calibration parameters. This method is favorable in the case of image pickup of a stationary object but requires a device for moving the camera accurately in order to track a moving image of a person or the like by means of a camera. There is also the problem that, because the calibration tool is installed and withdrawn, same is not suited to an outdoor application or the like.
Furthermore, a correction method in a case where a moving object is tracked is also illustrated in Japanese Patent Application Laid Open No. H5-241626 (‘Detected Position Correction method’, published on Sep. 21, 1993). When the details of Japanese Patent Application Laid Open No. H5-241626 are reviewed, calibration data (CDA), which is obtained by means of calibration in a position A prior to moving the camera, and calibration data (CDB) in position B after the camera has been moved by means of a calculation from movement amount data (CM) for the position and orientation of the camera are determined with the object of detecting the position of the target in every position and detecting the position of the target over a wide range by simply performing calibration once in an optional position. Next, the position of the target is detected from this calibration data (CDB) and from the imaging data (CM) for the target in position B of the camera. This technique involves establishing a correlation between the coordinates of the target and the coordinates of the camera also when the camera is moved once the calibration has been performed. However, the accuracy of this correlation is linked solely with the degree of accuracy of the movement amount data (CM) for the position and orientation of the camera. Therefore, a highly accurate drive mechanism must be provided. This also entails the fundamental defect that errors accumulate with each successive movement.